The insurance company said the notes will be offered in three series. The first $600 million will mature on Sept. 30, 2015 and bear interest at a fixed annual rate of 3.5 percent. A second $600 million will come due in September 2020 and carry a 5 percent annual interest rate. The final $300 million will mature in 2040 and carries a fixed annual interest rate of 6.25 percent.

The offering is expected to close on Friday.

Aon said net proceeds will be used in connection with the Hewitt acquisition. After the offering closes, the funds will be placed in an escrow account to be held until the deal is complete. The notes are being issued in lieu of drawing on a senior bridge term loan credit agreement that Aon executed last month.